Pressure sensing device for rotatably moving parts and pressure detection method therefor

ABSTRACT

A magnetic pressure sensing device for rotatably moving parts, of the type including at least one magnetic field source element associated to said rotatably moving part and a magnetic field sensing element associated to a fixed part to measure parameters of a magnetic field determined by said magnetic field source element, said parameters of the magnetic field being a function of the pressure applied to said rotatably moving part. The magnetic field source element includes means for rotating around at least one axis the direction of said magnetic field as a function of the pressure applied to said rotatably moving part.

The present invention relates to a magnetic pressure sensing device forrotatably moving parts, of the type comprising at least oneelectromagnetic field source element associated to said rotatably movingpart and a magnetic field sensor element associated to a fixed part tomeasure parameters of a magnetic field determined by said magnetic fieldsource element, said parameters of the magnetic field being a functionof the pressure applied to said rotatably moving part.

Measuring the pressure of rotating parts, such as tyres, has always beendifficult, due to the impossibility of wiring a sensor positioned on themoving part.

Of particular interest, for instance, is monitoring the pressure ofmotor vehicle tyres, even when said motor vehicles are moving, both forgeneral maintenance purposes, and for safety purposes, when the motorvehicle travels at high speed. It is therefore important that the driverbe aware at all times of the pressure of the tyres, which maydramatically influence the behaviour of the motor vehicle.

Several methods for monitoring tyre pressure and/or temperature areknown. Typically, a complex wiring technique is employed, or elsetransmitters and receivers of electromagnetic waves which require powersupplies and antennas.

French Patent no. 2 622 289 discloses a system for measuring pressure intyres, comprising means which are integral with the rotating part andgenerate a magnetic field that is variable as a function of the pressureof a compartment of the rotating part. An external magnetic sensormeasures the variable magnetic field during the cyclical passage infront of the fixed sensor.

The means that generate a variable magnetic field, operating by means ofa linear displacement as a function of the pressure of the magnetassociated to the rotating part, which determines a distance variationfrom the sensor on the fixed part and the consequent variation in theintensity of the measured magnetic field.

Such a system requires a considerable proximity of the sensor element tothe tread, to detect magnetic field intensity variations in a precisemanner.

The object of the present invention is to provide a solution capable offabricating a magnetic pressure sensing device for tyres whose precisionis influenced to little or no extent by the precision of the detectionof the intensity of the magnetic field.

According to the present invention, said object is achieved thanks to apressure sensing device, and a corresponding pressure measuring methodhaving the characteristics specifically set out in the claims thatfollow.

The invention shall now be described with reference to the accompanyingdrawings, provided purely by way of non limiting example, in which:

FIG. 1 shows an outline of the principle of operation of a magneticdevice according to the invention;

FIGS. 2A and 2B show in detail parts of the device of FIG. 1;

FIG. 3 shows an alternative embodiment of the device of FIG. 1;

FIG. 4 shows a second alternative embodiment of the device of FIG. 1.

The magnetic temperature sensor for rotatably moving parts proposedherein comprises at least one magnetic field source element associatedto said rotatably moving part, in a preferred version in an inner partof a tyre such as the inner tube, and a magnetic field sensing elementassociated to a fixed part, preferably the chassis of a motor vehicle,to measure parameters of the magnetic field of said source elements,said parameters of the magnetic field comprising in particular thedirection of said magnetic field, which is made a function of thepressure of said rotatably moving part through the adoption ofappropriate means of rotation as a function of pressure.

FIG. 1 shows a diagram of the principle of operation of a deviceaccording to the invention.

A tyre 11 rotating around an axis of rotation 12 comprises a magneticfield source 13, provided with its own magnetisation and able todetermine a magnetic field H.

Said magnetic field source 13 is preferably positioned inside the tyre11, in the inner tube. On a fixed part, in particular a fender 15 of amotor vehicle, not show in its entirety for the sake of simplicity, ispositioned a magnetic field sensor 14.

To the magnetic field H generated by the magnetic field source 13 in theregion between the tyre 11 and the fixed part 15 are associated a fieldintensity and a direction.

According to the invention, the magnetic field H of the magnetic fieldsource 13 can rotate and possibly also change its intensity as afunction of the pressure reached by the tyre 11. The magnetic fieldsensor 14 remotely measures the magnetic field H, thereby indirectlymeasuring pressure. The signal measured by the magnetic field sensor 14is then sent to an electronic unit of the motor vehicle, also not shownin FIG. 1, for processing and the generation of signals and alarms.

FIGS. 2A and 2B show in greater detail the magnetic field source 13associated to the tyre 11.

FIG. 2A shows the magnetic field source 13 in a first operatingconfiguration relating to a value of pressure P equal to P1, whilst FIG.2B shows the magnetic field source 13 in a second operatingconfiguration relating to a value of pressure P equal to P2, where P2 isgreater than P1.

Said magnetic field source 13 according to the invention comprises apermanent magnetic element 21, which generates a magnetic field H withconstant modulus and direction. Said permanent magnetic element 21,however, is advantageously associated to a rotating element 22, whichrotates according to a revolution motion along an axis 25, radialrelative to the circumference defined by the tyre 11.

As shown in FIG. 2, the rotating element 22 comprises a threaded bar 23,in practice a screw element, positioned along the axis 25 within acoaxial container cylinder 24, able to limit the vibrations produced bythe motion of the tyre 11.

The bar 23 is preferably fastened to a bottom 27 of the cylinder 24 onlyby means of its own distal end relative to the surface of the tyre 11,in such a way that it follows the axial displacements of said bottom 27,and therefore it is otherwise free to rotate within said cylinder 24following a thread. On the free end, and thus the proximal one relativeto the surface of the tyre 11, of said bar 23 is positioned thepermanent magnet 21, which, as shown in FIG. 2A, has its own two poles,and hence the associated magnetic field H, arranged according to adirection that is substantially tangential to the surface of the tyre11. In FIG. 2A, in particular, the position of the permanent magnet 21determines a magnetic field H that is tangential and aligned along thedirection of rotation of the tyre 11.

In FIG. 2B, as mentioned, the magnetic field source 13 is subjected to apressure P2 higher than the pressure P1. The bottom 27 of the cylinder24, which is made of resilient material, by virtue of the pressure riseis deformed, imparting a force along the axis 25 by effect of the thread26. In FIG. 2B, the permanent magnet 21 is rotated by substantially 90°and determines a magnetisation in substantially perpendicular directionto the direction of rotation of the tyre 11.

Clearly, the bottom 27 could alternatively be formed by means of apiston.

Though not shown in FIG. 2A or 2B, a spring is provided to return thebar 23 to the original resting position when pressure ceases. Thepressure measurement, therefore, derives from the resultant of theforces of pressure, elastic decompression of the spring and centrifugalforces of the massive movable parts. Lubricants can be used to minimisefriction between threaded bar 23 and thread 26.

The magnetic field sensor 14 indirectly measures the pressure P of thetyre 11, directly measuring the direction of the magnetic field Hdetermined by the magnetisation of the permanent magnet 21 positioned onthe rotating element 22.

The magnetic field sensor 14, which can be for instance a spin valvesensor, sensitive to switches in magnetic field direction, is able toperceive the variation in the direction of the magnetic field H, so itcan detect the pressure rise. In particular, appropriate circuits andmicro-controllers in the electronic unit can be employed to measure thenumber of rotations of the permanent magnet 21 which take place overtime, for instance by means of a simple counter, and hence determine thepressure of the tyre 17.

The permanent magnet 21 can be obtained by means of bulk magneticmaterials or hard ferromagnetic thin films plated by the correspondingthin film plating techniques such as sputtering or electroplating. Inthis case the permanent magnet 21 can comprise a single film or a stackof multiple films, as well as composite materials constituted byferromagnetic particles, having variable size (from nanometres tomillimetres) and shapes, incorporated and magnetised in a polymericmatrix. Particles can be synthesised on site or off-site in the polymer.

FIG. 3 shows an alternative embodiment of the sensor device according tothe invention, in which the magnetic field source 13 comprises arotating element 32 including a rotating disk 33, which is suspended bymagnetic levitation in a container 41 made of non-magnetic material, andset in rotation by a permanent magnet 34, which is associated to adeformable elastic membrane 35. In particular, the container 41superiorly comprises a housing 43 for the disk 33 and inferiorly definesa chamber 42, closed inferiorly by the membrane 35. The chamber 42 andthe housing 43 are separated by a wall 44.

The pressure P, external to the chamber 42, acts on the membrane 35,moving in particular, in the direction of an axis 45 perpendicular tothe surface of the disk 33, the centre of said membrane 35 whereon ispositioned said permanent magnet 34, which comprises sectors withopposite polarity, in such a way as to set in rotation the magnetic disk33 when it approaches it by effect of the pressure P.

When the pressure P on the membrane 35 decreases, the permanent magnet34 moves away and the disk 33 can tend to a resting position thanks toappropriate return permanent magnets 37, which substantially operate asmagnetic springs.

The rotating disk 33 in turn comprises magnetic sectors 39, positionedon its outermost circumference.

Also provided are magnetic suspensions 38, formed by means ofappropriate magnets with opposite polarity, which therefore repel eachother, positioned in pairs on the disk 33 and on the container 41, toallow the disk 33 to levitate and rotate in the absence of friction.

The central area of the disk 33 bears a permanently magnetised area 40,which generates the magnetic field H that is measured by the magneticfield sensor 14. The magnetic field sensor 14 in this case needs to besensitive only to variations in the direction of the magnetisation ofsaid permanently magnetised area 40.

The interior of the chamber 42 may be pressurised to a referencepressure P_(ref), in such a way that the membrane 35 is deflected onlyafter the pressure P is greater than said reference pressure P_(ref).

It should be noted that, in the sensor device of FIG. 3, the centrifugalforce on the tyre 11 acts in the same direction as the pressure P. Saidcentrifugal force can therefore manifest itself as an error in thepressure reading and thus needs compensation.

The effect of centrifugal force can be minimised by providing movableparts with defined ratios between mass and dimensions, in particular themembrane 35 and the magnetic disk 33 for this purpose must have areduced and a large surface. In fact, the pressure P determines a forcewhich is proportional to the surface, whilst centrifugal force is afunction of mass.

Additionally, in the case of a tyre 11 of a motor vehicle, one canexploit, when present, the ABS braking control system, which for itsoperation measures the velocity of rotation of the wheel. From saidmeasurement of the velocity of the wheel, therefore, it is possible tocalculate centrifugal force and correct, for instance in the electronicunit of the motor vehicle, for each velocity of rotation the pressuremeasurement obtained.

FIG. 4 shows a schematic view of an additional embodiment, in which themagnetic field sensor 14 positioned on the fixed part 15 is sensitivesolely to the direction of the magnetic field H. The magnetic fieldsource 13 in this case comprises a container 51 made of non-magneticmaterial which defines a single chamber 52 under reference pressureP_(ref), delimited inferiorly by a resilient membrane 55 and superiorlyby a wall 64. Said membrane 55 bears a permanent magnet 54 having adirection of magnetisation M that is substantially parallel to thesurface of the membrane. Similarly to the device of FIG. 3, the pressureP is exerted on the outer surface of said membrane 55, along an axis 65.The container 51 in the wall 64 positioned superiorly to the chamber 52has an anisotropic ferromagnetic layer 53, whose magnetisation, at restand in the absence of applied pressure is oriented along the axisorthogonal to the direction of magnetisation M of the permanent magnet54 lying on the membrane 55.

When, by effect of the pressure P, the membrane 55 with its permanentmagnet approaches the anisotropic ferromagnetic layer 53, the permanentmagnet 54 on the membrane 55 tends to influence, and then rotate thedirection of magnetisation of the anisotropic ferromagnetic layer 53,and hence the direction of the magnetic field H, making it parallel toits own magnetisation. FIG. 4 shows said operating configuration, inwhich the field H produced by the anisotropic ferromagnetic layer 53 isparallel to the direction of magnetisation M of the permanent magnet 54,since said permanent magnet 54 has moved closer under the effect of thepressure P.

Thus, a displacement of the permanent magnet 54 along the axis 65 byeffect of the pressure P exerted on the membrane 55 determines arotation of the magnetic field H originated by the anisotropicferromagnetic layer 53, which can be measured by the magnetic fieldsensor 14.

Said rotation of the magnetic field H of the anisotropic ferromagneticlayer 53 is generally not a sudden transition with respect to the changein the distance from the permanent 54; rather, it is usually acontinuous function of distance, since the magnetic domains of theanisotropic ferromagnetic layer 53 do not all switch together. It istherefore possible, by measuring the angle of the direction of themagnetic field H, to have a continuous measurement of the distance ofthe permanent magnet 54 relative to the anisotropic ferromagnetic layer53, and hence a continuous measurement of the exerted pressure P.

The anisotropic ferromagnetic layer 53 can be obtained by means of acomposite structure of magnetic particles incorporated in a matrix.

The membrane 55 can be constituted by a composite elastomeric material,in which the oriented magnetic particles incorporated in an elastomerprovide the membrane with magnetic properties whilst altering itselastic properties only to a minimal extent. The magnetic particles canbe created on-site during the formation of the polymer, or incorporatedsubsequently. The elastomer is made reticular in a magnetic field. Themagnetic membrane therefore behaves like an elastic permanent magnet.

The solution described above allows to achieve considerable advantagesover prior art solutions.

The pressure sensing device according to the invention advantageouslyallows to correlate the pressure with rotations in the direction of themagnetic field, so that the correct determination of the intensity ofsaid magnetic field to the sensor has less influence, because thetransitions determined by the rotating element can be measured.

Naturally, without altering the principle of the invention, theconstruction details and the embodiments may vary widely relative towhat is described and illustrated purely by way of example herein,without thereby departing from the scope of the present invention.

It is possible to insert a permanent magnet whose magnetisation modulusvaries even considerably with pressure.

It must also be kept in mind that there may be combinations of thedifferent embodiments described above for the magnetic field sourceelement and, in particular, there may be multiple elements, of the sametype or of different types, located on the rotatably moving part. Thesignals measured by one or more sensors on the fixed part, whichrepresent a composite information about pressure, can then beconveniently analysed and processed.

The magnetic field sensor can be obtained with any digital or analoguemagnetic field sensor, such as a simple solenoid, or an AMR (AnisotropicMagnetic Resistance), Hall, GMR (Giant Magnetic Resistance), TMR (Tunneljunction Magneto Resistance) sensor.

A pressure sensor device of the type described above can be used in avariety of applications requiring the measuring of a pressure.

In relation to measuring pressure in a tyre, the pressure sensor may bea part of an appropriate measuring unit, further comprising tyre wearsensors and/or temperature sensors and, possibly, actuators or valves tore-establish tyre pressure, said unit being located directly on the tyreand powered independently through the conversion of vibration energyderiving from the motion of the tyre.

In particular, it is possible to exploit the sensor positioned on thefixed part to detect magnetic field variations in intensity and/ordirection of other magnetic field sources under the control of otherquantities, such as temperature.

However, it is clear that the proposed device can be applied in allpressure measurements compatible with a magnetic pressure sensing devicelike the one described herein, which includes at least one magneticfield source element associated to a rotatably moving part and amagnetic field sensing element associated to a fixed part to measureparameters of a magnetic field determined by said magnetic field sourceelement, said parameters of the magnetic field being a function of thepressure of said rotatably moving part, where the magnetic field sourceelement comprises means for rotating the direction of the emittedmagnetic field along at least one axis.

1. A magnetic pressure sensing device for rotatably moving parts, of thetype comprising at least one magnetic field source element associated tosaid rotatably moving part and a magnetic field sensing elementassociated with a fixed part to measure parameters of a magnetic fielddetermined by said magnetic field source element, said parameters of themagnetic field (H) being a function of the pressure applied to saidrotatably moving part, wherein said magnetic field source elementcomprises means for rotating along at least one axis the direction ofsaid magnetic field as a function of the pressure applied to saidrotatably moving part.
 2. A device as claimed in claim 1, wherein saidmeans for rotating around at least one axis the direction of saidmagnetic field comprise means for converting a force determined by saidpressure applied along said axis into a rotation of the direction ofsaid magnetic field.
 3. A device as claimed in claim 1, wherein saidmagnetic field source element comprises at least a first permanentmagnet for generating the magnetic field.
 4. A device as claimed inclaim 2, wherein said means for converting a force determined by saidpressure applied along said axis into a rotation of the direction ofsaid magnetic filed comprise means for converting said force into adisplacement along said axis.
 5. A device as claimed in claim 4, whereinsaid means for converting said force into a displacement along said axiscomprise a resilient membrane.
 6. A device as claimed in claim 5,wherein said resilient membrane is associated a second permanent magnet.7. A device as claimed in claim 6, wherein said first permanent magnetis associated to a levitating disk able to be set in rotating motion bysaid second permanent magnet.
 8. A device as claimed in claim 6, whereinsaid first permanent magnet is able to align the direction of a magneticfiled which emits to the direction of a magnetization of the secondpermanent magnet as a function of the distance between said first andsecond permanent magnet.
 9. A device as claimed in claim 4, wherein saidmeans for converting a force determined by said pressure along said axisinto a rotation of the direction of said magnetic field comprise a screwrotating element associated to said first permanent magnet.
 10. A deviceas claimed in claim 1, wherein said parameters of the magnetic filedmeasured by the magnetic filed sensor comprise the direction and/or theintensity of the magnetic filed.
 11. A device as claimed in claims 1,wherein said magnetic field sensor is able to measure the magnetic filedparameters of additional sources of magnetic field, variable accordingto a physical quantity and associated to said rotatably moving part, inparticular said physical quantity being a temperature.
 12. A device asclaimed in claim 1, wherein said moving part is a tyre for motorvehicle.
 13. A device as claimed in claim 1, wherein it is comprised ina measuring unit, further comprising tyre wear sensors and/ortemperature sensors and, possibly actuators or valves to re-establishthe pressure of the tyre, said unit being positioned directly on thetyre.
 14. A device as claimed in claim 1, wherein said first permanentmagnet and/or second permanent magnet are formed with bulk magneticmaterials or hard ferromagnetic thin films or composite materialsconstituted by ferromagnetic particles incorporated and magnetised inpolymeric matrix.
 15. A device as claimed in claim 5, wherein saidresilient membrane corresponds, partly or completely, to said secondpermanent magnet.
 16. A device as claimed in claim 1, wherein saidmagnetic field sensor is a spin valve device.
 17. A method for measuringthe pressure of rotatably moving parts using the device as claimed inclaim 1, wherein it associates variations in pressure to variations inthe direction of the magnetic field generated by the magnetic fieldsource element and measured by the magnetic field sensor.
 18. A methodas claimed in claim 17, wherein it further associates variations inpressure to variations in the intensity of the magnetic field generatedby the magnetic field source element and measured by the magnetic fieldsensor.